U.S. patent application number 10/698214 was filed with the patent office on 2005-05-05 for vibrating beam accelerometer two-wafer fabrication process.
This patent application is currently assigned to Honeywell International, Inc.. Invention is credited to Yu, Lianzhong.
Application Number | 20050091843 10/698214 |
Document ID | / |
Family ID | 34550573 |
Filed Date | 2005-05-05 |
United States Patent
Application |
20050091843 |
Kind Code |
A1 |
Yu, Lianzhong |
May 5, 2005 |
Vibrating beam accelerometer two-wafer fabrication process
Abstract
A method for fabrication of microelectromechanical systems
(MEMS) integrated micro devices and acceleration sensor devices
formed according to the method, the method being micromachining an
array of first three-dimensional micromechanical device features in
a first silicon wafer; micromachining an array of second
three-dimensional micromechanical device features in a second
silicon wafer, wherein the second three-dimensional micromechanical
device features are configured to cooperate with the first
three-dimensional micromechanical device features when joined
therewith; mutually aligning the first and second arrays of device
features by aligning the first and second wafers; permanently
joining the first and second arrays of device features into an
array of integrated micro devices as a function of permanently
joining the first and second wafers into a single composite wafer;
and subsequently separating the array of integral devices into
individual devices each having a set of the first and second device
features.
Inventors: |
Yu, Lianzhong; (Redmond,
WA) |
Correspondence
Address: |
HONEYWELL INTERNATIONAL INC.
101 COLUMBIA ROAD
P O BOX 2245
MORRISTOWN
NJ
07962-2245
US
|
Assignee: |
Honeywell International,
Inc.
Morristown
NJ
07962
|
Family ID: |
34550573 |
Appl. No.: |
10/698214 |
Filed: |
October 31, 2003 |
Current U.S.
Class: |
29/830 ; 438/50;
73/514.29 |
Current CPC
Class: |
B81C 2201/019 20130101;
Y10T 29/49126 20150115; B81C 2203/051 20130101; G01P 15/0802
20130101; G01P 15/097 20130101; B81C 3/002 20130101; B81B 2201/0235
20130101; Y10T 29/49155 20150115 |
Class at
Publication: |
029/830 ;
073/514.29; 438/050 |
International
Class: |
H01L 021/304; G01P
015/097 |
Claims
What is claimed is:
1. A method for fabrication of microelectromechanical systems
(MEMS) integrated micro devices, the method comprising:
micromachining an array of first three-dimensional micromechanical
device features in a first silicon wafer; micromachining an array
of second three-dimensional micromechanical device features in a
second silicon wafer, the second three-dimensional micromechanical
device features being configured to cooperate with the first
three-dimensional micromechanical device features when joined
therewith; mutually aligning the first and second arrays of device
features; permanently joining the first and second arrays of device
features into an array of integrated micro devices as a function of
permanently joining the first and second silicon wafers into a
single composite wafer; and subsequently separating the array of
integral devices into individual devices each having a set of the
first and second device features.
2. The method of claim 1 wherein micromachining an array of first
three-dimensional micromechanical device features in a first
silicon wafer further comprises micromachining an array of both
partial and complete stand-alone three-dimensional micromechanical
device features; and micromachining an array of second
three-dimensional micromechanical device features in a second
silicon wafer further comprises micromachining an array of both
partial and complete stand-alone three-dimensional micromechanical
device features, the partial device features in the second silicon
wafer being configured to be joined with the partial device
features in the first silicon wafer.
3. The method of claim 2 wherein permanently joining the first and
second arrays of device features into an array of integrated micro
devices further comprises joining the partial device features in
the first and second silicon wafers into complete composite device
features.
4. The method of claim 3 wherein micromachining an array of both
partial and complete stand-alone three-dimensional micromechanical
device features in a first silicon wafer further comprises
micromachining an array of first partial proof masses each
connected to a first partial frame by one or more complete
stand-alone flexures; and micromachining an array of both partial
and complete stand-alone three-dimensional micromechanical device
features in a second silicon wafer further comprises micromachining
an array of second partial proof masses each connected to a second
partial frame by one or more complete stand-alone vibratory beams;
and wherein the first and second partial proof masses and the first
and second partial frames are mutually configured and arranged
relative to the first and second wafers for permanently joining
into an array of composite stand-alone three-dimensional
micromechanical device features having a composite proof mass each
connected to a composite frame by the one or more flexures and the
one or more vibrating beams.
5. The method of claim 4 wherein machining the first partial frames
further comprises machining a relief arranged to cooperate with
each of the one or more vibrating beams.
6. The method of claim 4, further comprising: determining for a
plurality of the first wafers a minimum yield of the first
three-dimensional micromechanical device features micromachined
therein; determining for a plurality of the second wafers a minimum
yield of the second three-dimensional micromechanical device
features micromachined therein; and wherein each of mutually
aligning the first and second arrays of device features and
permanently joining the first and second arrays of device features
into an array of integrated micro devices further comprises using
one of the first wafers determined to have a minimum yield of the
first three-dimensional micromechanical device features
micromachined therein and one of the second wafers determined to
have a minimum yield of the second three-dimensional
micromechanical device features micromachined therein.
7. The method of claim 4, further comprising providing means for
vibrating each of the one or more vibratory beams at a respective
resonant frequency when the composite proof mass is at rest.
8. A method for fabrication of microelectromechanical systems
(MEMS) integrated micro devices, the method comprising: forming an
array of first three-dimensional micromechanical device features in
each of a plurality of first silicon wafers each having top and
bottom substantially parallel surfaces spaced apart by a thickness
of the first silicon wafer material; applying a first alignment
mark to a face of each of the first wafers relative to the array of
first device features; in each of a plurality of second silicon
wafers each having top and bottom substantially parallel surfaces
spaced apart by a thickness of the second silicon wafer material,
forming an array of second three-dimensional micromechanical device
features that includes one or more device features that are
different from one or more of the first device features formed in
the first wafers and are configured to cooperate with different
ones of the first device features formed in the first wafers;
applying a second alignment mark to a face of each of the second
wafer relative to the array of second device features; preparing
one of each of the first and second wafers for wafer bonding;
mutually aligning the first and second wafers as a function of
aligning the respective first and second alignment marks;
permanently bonding the first and second wafers into a single
composite wafer having the first device features formed in the
first wafer permanently bonded to the second device feature in the
second wafer configured to cooperate therewith.
9. The method of claim 8 wherein permanently bonding the first and
second wafers into a single composite wafer further comprises
high-temperature silicon fusion bonding the first and second
wafers.
10. The method of claim 8 wherein: forming an array of first
three-dimensional micromechanical device features in the first
silicon wafers further comprises forming an array of first
three-dimensional proof masses suspended for motion relative to
first frames by one or more flexures; forming an array of second
three-dimensional micromechanical device features in the second
silicon wafers further comprises forming an array of second
three-dimensional proof masses suspended relative to second frames
by one or more vibratory beams; and permanently bonding the first
and second wafers into a single composite wafer further comprises
permanently bonding respective first and second three-dimensional
proof masses into single composite three-dimensional proof masses,
and permanently bonding respective first and second
three-dimensional frames into single composite three-dimensional
frames suspended from respective composite proof masses by
respective flexures and coupled thereto by respective vibratory
beams.
11. The method of claim 10, further comprising forming a plurality
of electrical conductors over each of the vibratory beams,
including wire bond pads electrically coupled to the electrical
conductors.
12. A composite microelectromechanical system (MEMS) integrated
micro acceleration sensor, comprising a substrate having formed
therein a proof mass suspended from a frame by a flexure and a
vibratory beam coupled between the proof mass and frame, wherein:
at least one of the proof mass and the frame further comprises a
composite of two or more substrates, and at least one of the
flexure and the vibratory beam is formed complete in a single one
of the two or more substrates.
13. The acceleration sensor of claim 12 wherein the flexure is
formed complete in a single one of the two or more substrates.
14. The acceleration sensor of claim 12 wherein the vibratory beam
is formed complete in a single one of the two or more
substrates.
15. The acceleration sensor of claim 12 wherein the flexure is
formed complete in a single one of the two or more substrates, and
the vibratory beam is formed complete in a different single one of
the two or more substrates.
16. The acceleration sensor of claim 15 wherein both the proof mass
and the frame further comprises a composite of two or more
substrates.
17. The acceleration sensor of claim 16, further comprising means
for causing the vibratory beam to vibrate at a resonant frequency
when the composite proof mass is at rest relative to the composite
frame.
18. A composite microelectromechanical system (MEMS) integrated
micro acceleration sensor, comprising: first and second silicon
substrates each having top and bottom substantially parallel
surfaces spaced apart by the thickness of the silicon substrate
material; a composite proof mass partially formed in each of the
first and second substrates and permanently joined together; a
composite frame partially formed in each of the first and second
substrates and a flexure suspending the composite proof mass for
movement relative to the composite frame; a vibratory beam coupled
between the composite proof mass and the composite frame;
electrical means for vibrating the vibratory beam at a resonant
frequency when the composite proof mass is at rest relative to the
composite frame; and electrical means for receiving a signal
representative of a variation in the resonant frequency of the
vibrating beam as a function of tension or compression forces being
applied by movement of the composite proof mass relative to the
composite frame.
19. The acceleration sensor of claim 18 wherein the flexure
suspending the composite proof mass from the composite frame
further comprises a flexure being completely formed in one of the
first and second substrates integrally with one portion of the
composite proof mass and integral with one portion of the composite
frame.
20. The acceleration sensor of claim 18 wherein the vibratory beam
coupled between the composite proof mass and the composite frame
further comprises a vibratory beam being completely formed in one
of the first and second substrates integrally with one portion of
the composite proof mass and integral with one portion of the
composite frame.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to micromachined accelerometer
devices and methods, and in particular to double-sided
accelerometer devices using high resolution vibratory resonators
and methods for manufacturing the same.
BACKGROUND OF THE INVENTION
[0002] As described by Norling in U.S. Pat. No. 5,367,217, FOUR BAR
RESONATING FORCE TRANSDUCER," the complete disclosure of which is
incorporated herein by reference, two- and four-bar vibrating beam
force sensing elements have been used as vibratory resonators in
crystal-controlled oscillators and are generally known in the art.
Such force sensing elements have been known to be used in various
transducers to measure various parameters, including acceleration,
force, temperature, pressure and weight. In particular, such
vibrating beam force sensing elements are responsive to forces,
such as longitudinal or axial forces, which cause a variation of
the frequency of vibration of the beams that, in turn, cause a
variation in an output frequency of the oscillator which can be
used as a measure of the applied force.
[0003] In U.S. Pat. No. 5,501,103, "TWO-PORT ELECTROMAGNETIC DRIVE
FOR A DOUBLE-ENDED TUNING FORK," the complete disclosure of which
is incorporated herein by reference, Woodruff, et al. describes a
vibrating beam accelerometer which includes two- and four-bar
electromagnetically excited double-ended tuning forks (DETF) of the
type described by Norling. As described by Woodruff, et al.,
vibrating beam accelerometers are generally known in the art. The
DETF of Woodruff is formed with separate conducting paths for the
drive circuit and pick-off circuit that minimize problems
associated with vibrating beam accelerometers formed with a single
conducting path resulting from variations of the resistance path
due to manufacturing tolerances and temperature changes.
[0004] FIG. 1 illustrates one example of a known vibrating beam
accelerometer 1. Such known vibrating beam accelerometers normally
include a pendulum or proof mass 3, connected to a frame 5 by way
of a pair of flexures 7 to enable the proof mass 3 to rotate about
a hinge axis HA, defined by the flexures 7. A double-ended tuning
fork (DETF) having two or four vibrating beams is connected between
the frame 5 and the proof mass 3, perpendicular to the hinge axis
HA to define a sensitive axis SA. In the example of FIG. 1, a DETF
9 having four vibrating beams 11, 13, 15 and 17 is connected
between the frame 5 and the proof mass 3. Excitation is applied to
the DETF 9 to cause the vibrating beams 11, 13, 15 and 17 to
vibrate at a resonant frequency when the proof mass 3 is at rest.
Forces applied along the sensitive axis SA apply either tension or
compression forces the vibrating beams 11, 13, 15 and 17 which
changes their resonant frequency. This change in frequency, in
turn, is a measure of the force applied along the sensitive axis
SA.
[0005] Various types of excitation are known to force the vibrating
beams 11, 13, 15 and 17 to vibrate, such as electromagnetic,
electrostatic, and thermal excitation. The type of excitation
depends on the particular materials used for construction. For
example, crystalline quartz DETFs are excited according to the
piezoelectric property of the quartz. Silicon DETFs are normally
micromachined and are excited by other means, such as
electrostatically or electromagnetically.
[0006] FIG. 1 illustrates an exemplary silicon micromachined
vibrating beam accelerometer that includes a four-beam double-ended
tuning fork that is adapted to be excited electromagnetically. In
such an embodiment, the outer pair of vibrating beams 11 and 17 are
electrically connected together by a conducting member 19. The
inner pair of vibrating beams 13 and 15 is connected together by a
conducting member 21. Free ends of each of the vibrating beams 11,
13, 15 and 17 are connected to wire bond pads 23, 25, 27 and 29. In
such an embodiment, a conductive material, such as gold, is applied
to the vibrating beams 11, 13, 15 and 17 as well as the wire bond
pads 23, 25, 27 and 29 to enable electric current to flow between
the wire bond pads 23, 25, 27 and 29 through the respective
vibrating beams 11, 13, 15 and 17. Such a configuration provides
separate conducting paths between the vibrating beams used for the
drive circuit and the vibrating beams used for the pick-off
voltage. In particular, a first conducting path is formed between
the outer pair of beams 11 and 17, while a second conducting path
is provided between the inner pair of tines 13 and 15.
[0007] An oscillator circuit is provided to drive the vibrating
beams. In particular, the outer pair of vibrating beams 11, 17 are
used in the drive circuit, while the inner pair of vibrating beams
13 and 15 are used in the pick-off circuit. Referring first to the
drive circuit, the electrode 29 is connected to ground by way of an
electrical conductor 31. The other inner vibrating beam 13 is
connected to an amplifier 33 by way of an electrical conductor 35
connected to the electrode 25. The output of the amplifier 33 is,
in turn, connected to an amplitude limiter 37 by way of an
electrical conductor 39. The output of the amplitude limiter 37, in
turn, is used to provide an alternating current (AC) drive current
of the outer pair of vibrating beams 11, 17. In particular, the
output of the amplitude limiter 37 is connected to the electrode 27
by way of an electrical conductor 41. This forces the drive current
up the outer beam 17 and down the outer beam 11 to ground. The
drive current is then connected to ground by way of the electrical
conductor 43. An externally generated magnetic field B is applied
in a direction generally perpendicular to the plane of the DETF 9.
The magnetic field B having flux lines in a direction generally
perpendicular to the plane of the DETF, interacts with the AC drive
current in the outer beams 11 and 17 which causes these beams 11,
17 to vibrate. Mechanical couplings between the pair of beams 11
and 13 and between the pair of beams 15 and 17 are provided by
respective cross members 45 and 47. More particularly, one cross
member 45 is connected between the tine 11 and 13 to cause these
beams to vibrate together. The other cross member 47 is connected
between the beams 15 and 17 to cause them to vibrate together.
Since the cross member 45 is connected between the beams 11 and 13
and the cross member 47 is connected between the beams 15 and 17,
all four beams 11, 13, 15 and 17 are mechanically coupled together
forming a two degree of freedom mechanical system. These mechanical
couplings of the inner pair of beams 13 and 15 relative to the
outer pair of beams 11 and 17 cause a voltage to be generated
across the inner pair of beams 13 and 15. This voltage is generated
across the electrodes 25 and 29. This voltage, known as the pickoff
voltage, is then applied to an amplifier 33 by way of a positive
feedback loop in order to form an oscillator.
[0008] In operation, in response to an excitation or drive current
the beams 11, 13, 15 and 17 are forced to vibrate at a resonant
frequency while the proof mass 3 is at rest. Force applied to the
proof mass 3 along the sensitive axis SA causes the vibrating beams
11, 13, 15 and 17 to undergo either tension or compression which,
in turn, causes a variation in the resonant frequency at which the
beams 11, 13, 15 and 17 vibrate. This variation in the resonant
frequency is useful a measure of the applied force. This frequency
can be measured at the output of the amplifier 33 along a signal
line 49 by any conventional frequency measuring circuitry which is
well known in the art. According to known prior art, the vibrating
beam accelerometer device 1 optionally includes a second DETF
sensor 9' coupled between the an end of the proof mass 3 and frame
5 opposite from the suspension flexures 7. Such a dual DETF
arrangement provides many advantages such as doubling the output
and common mode cancellation of error sources, which is the
tracking and mutual cancellation of the common mode responses of
two DETFs in a single sensor.
[0009] Vibrating beam accelerometer of the type depicted in FIG. 1
have been fabricated from a body of semiconductor material, such as
silicon, using micromachining techniques as microelectromechanical
systems, or "MEMS," integrated micro devices or systems combining
electrical and mechanical components fabricated using integrated
circuit (IC) batch processing techniques.
[0010] In the most general form, MEMS consist of mechanical
microstructures, microsensors, microactuators and electronics
integrated in the same environment, i.e., on a silicon chip. MEMS
is an enabling technology in the field of solid-state transducers,
i.e., sensors and actuators. The microfabrication technology
enables fabrication of large arrays of devices, which individually
perform simple tasks but in combination can accomplish complicated
functions. Current applications include accelerometers, pressure,
chemical and flow sensors, micro-optics, optical scanners, and
fluid pumps. For example, one micromachining technique involves
masking a body of silicon in a desired pattern, and then deep
etching the silicon to remove unmasked portions thereof. The
resulting free-standing three-dimensional silicon structure
functions as a miniature mechanical force sensing device, such as
an accelerometer that includes a proof mass suspended by a flexure.
Existing techniques for manufacturing these miniature devices are
described in U.S. Pat. Nos. 5,006,487, "Method of Making an
Electrostatic Silicon Accelerometer" and 4,945,765 "SILICON
MICROMACHINED ACCELEROMETER," the complete disclosures of which are
incorporated herein by reference.
[0011] Vibrating beam accelerometer of the type depicted in FIG. 1
have different features provided in the front and back surfaces 50,
51. For example, features of the DETF 9 are provided in the front
surface 50 while features of the suspension flexures 7 are formed
in the back surface 51. Such two-sided structures have been formed
using different techniques with different results. For example, a
large array of the moving system (proof mass 3, frame 5, suspension
flexures 7, etc.) is fabricated in one wafer substrate while
another large array of the DETF sensors 9 is fabricated in a
different wafer substrate. The DETF sensors 9 are then attached
between proof mass and frame, as by an adhesive or other bonding
agent. A vibrating beam accelerometer having such an adhesively
bonded DETF sensor is described by Woodruff, et al. in U.S. Pat.
No. 6,484,578, "VIBRATING BEAM ACCELEROMETER." While effective for
some applications this bonding of the DETF sensor 9 introduces an
area of inherent thermal mismatch that leads to inaccuracies in the
sensor output that is unacceptable in high accuracy applications.
Furthermore, because the features, particularly the DETF sensors 9,
are so small and delicate, adhesive or other bonding of the DETF
sensors 9 is not known to be feasible using today's assembly
techniques. Rather, the method of forming two-sided structures by
adhesive or other bonding of the DETF sensors 9 is currently
feasible only in quartz because the structures are larger and
stronger than those formed in silicon, and are therefore more
easily manipulated.
[0012] One effective alternative is fabricating a large array of
features in a first side, such as the features of the DETF 9 in the
front side 50, of a silicon wafer, then masking these front side
features to protect them from further etching, and only then
fabricating a matching array of the backside features, such as the
suspension flexures 7, in the second side of the wafer. This
process of etching the entire array of devices from both sides of
the wafer is effective for providing all the device's features
integrally in a single substrate without introducing adhesives
other bonding agents so that a highly accurate device results. One
drawback to such double-sided fabrication is yield which requires
high yields of both the first- and second-side features. For
example, even masking the first side cannot prevent some damage of
the first-side features so that overall yield suffers even more,
regardless of the yield of second-side features. Additionally, high
yields of the first-side processing requires high yields of the
second-side processing as well, else the first-side processing is
wasted. In practice, even if the yield of the first-side processing
is 70 to 90 percent, if yield of the second-side processing is only
10 percent, the yield for the entire batch is no more than 10
percent. Obviously, a low yield of the first-side results in scrap
of the entire batch and second side processing does not occur.
[0013] Therefore, a more reliable double-sided fabrication process
is desirable.
SUMMARY OF THE INVENTION
[0014] A reliable double-sided fabrication process for
microelectromechanical system (MEMS) integrated micro devices is
provided that overcomes limitations of the prior art, the method
including forming a large array of first three-dimensional
micromechanical device features in each of several first silicon
wafers wherein each of the first silicon wafers has top and bottom
substantially parallel surfaces spaced apart by a thickness of the
first silicon wafer material. In each of several second silicon
wafers wherein each of the each has top and bottom substantially
parallel surfaces spaced apart by a thickness of the second silicon
wafer material that may be different from the thickness of the
first silicon wafer, forming an array of second three-dimensional
micromechanical device features that includes one or more device
features that are different from one or more of the first device
features formed in the first wafers and are configured to cooperate
with different ones of the first device features formed in the
first wafers. According to the invention, the method includes
applying a first alignment mark to a face of each of the first
wafers relative to the array of first device features; and applying
a second alignment mark to a face of each of the second wafer
relative to the array of second device features. The invention
includes thereafter preparing one of each of the first and second
wafers for wafer bonding using conventional preparation techniques
appropriate to the type of wafer bonding to be used. After
preparing the wafers for bonding, the invention includes mutually
aligning the first and second wafers as a function of aligning the
respective first and second alignment marks; and then assembling
and permanently bonding the first and second wafers into a single
composite wafer having the first device features formed in the
first wafer permanently bonded to the second device feature in the
second wafer configured to cooperate therewith.
[0015] According to another aspect of the invention, the
permanently bonding of the first and second wafers into a single
composite wafer further includes bonding the first and second
wafers using a conventional high-temperature silicon fusion bonding
process.
[0016] According to another aspect of the invention, the forming of
the array of first three-dimensional micromechanical device
features in the first silicon wafers includes forming an array of
first three-dimensional pendulums or proof masses suspended for
translational or rotational motion relative to first frames by one
or more flexures; forming an array of second three-dimensional
micromechanical device features in the second silicon wafers
includes forming an array of second three-dimensional proof masses
suspended relative to second frames by one or more vibratory beams;
and permanently bonding the first and second wafers into a single
composite wafer includes permanently bonding respective first and
second three-dimensional proof masses into single composite
three-dimensional proof masses, and permanently bonding respective
first and second three-dimensional frames into single composite
three-dimensional frames suspended from respective composite proof
masses by respective flexures and coupled thereto by respective
vibratory beams.
[0017] According to another aspect of the invention, the method of
the invention also includes forming electrical conductors over each
of the vibratory beams, including wire bond pads electrically
coupled to the electrical conductors.
[0018] According to yet other aspects of the invention, a composite
microelectromechanical system (MEMS) integrated micro acceleration
sensor is provided in a substrate having formed therein a proof
mass suspended from a frame by a flexure and a vibratory beam
coupled between the proof mass and frame, wherein at least one of
either the proof mass or the frame is formed of a composite of two
or more substrates, and at least one of either the flexure or the
vibratory beam is formed complete in a single one of the two or
more substrates.
[0019] According to another aspect of the invention, the flexure of
the acceleration sensor is formed complete in a single one of the
two or more substrates.
[0020] According to another aspect of the invention, the vibratory
beam of the acceleration sensor is formed complete in a single one
of the two or more substrates.
[0021] According to another aspect of the invention, the flexure is
formed complete in a single one of the two or more substrates, and
the vibratory beam is formed complete in a different single one of
the two or more substrates.
[0022] According to another aspect of the invention, both the proof
mass and the frame are formed of a composite of two or more
substrates.
[0023] According to another aspect of the invention, the
acceleration sensor includes electrical means for causing the
vibratory beam to vibrate at a resonant frequency when the
composite proof mass is at rest relative to the composite frame,
and also includes electrical means for receiving a signal
representative of a variation in the resonant frequency of the
vibrating beam as a function of tension or compression forces being
applied by movement of the composite proof mass relative to the
composite frame.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
becomes better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0025] FIG. 1 illustrates one example of a known vibrating beam
accelerometer;
[0026] FIG. 2 illustrates the method of the invention embodied in a
process flow diagram;
[0027] FIG. 3 illustrates the method of the invention embodied in a
composite wafer having a large array of composite MEMS devices
formed therein;
[0028] FIG. 4 illustrates the method of the invention embodied in
an exemplary composite MEMS device;
[0029] FIG. 5 illustrates a first portion of the method of the
invention embodied in an exemplary first substrate having both
partial and complete stand-alone 3-dimensional features formed
therein; and
[0030] FIG. 6 illustrates a second portion of the method of the
invention embodied in an exemplary second substrate having both
partial and complete stand-alone 3-dimensional features formed
therein, the partial and complete 3-dimensional features being
configured and arranged to cooperate with those of the first
substrate illustrated in FIG. 5 to form composite and independent
cooperating 3-dimensional features in a composite MEMS device.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0031] In the Figures, like numerals indicate like elements.
[0032] The present invention is a method for fabrication of
microelectromechanical systems, or "MEMS," integrated micro
devices, including the steps of micromaching a patterned array of
first three-dimensional (hereinafter 3-dimensional) micromechanical
device features in a first silicon wafer; micromaching a first
optical alignment mark on a face of the first wafer relative to the
array of first device features; micromaching a patterned array of
second 3-dimensional micromechanical device features in a second
silicon wafer, the second 3-dimensional micromechanical device
features being configured to cooperate with the first 3-dimensional
micromechanical device features when joined therewith; micromaching
a second optical alignment mark on a face of the second wafer
relative to the array of second device features; aligning the first
patterned array of device features relative to the second patterned
array of device features as a function of aligning the first and
second optical alignment marks; permanently joining the first and
second arrays of device features into an array of integral devices
as a function of permanently joining the first and second silicon
wafers into a single composite wafer; and subsequently separating
the array of integral devices into individual devices each having a
set of the first and second device features.
[0033] Additionally, the method of the invention is embodied in a
number of the individual micromachined devices formed according to
the method of the invention. Accordingly, by example and without
limitation, the individual micromachined devices of the invention
are embodied in individual micromachined vibrating beam
accelerometers fabricated as microelectromechanical systems, or
"MEMS," integrated micro devices having a set of 3-dimensional
micromechanical device features formed in a first silicon
substrate; a second set of 3-dimensional micromechanical device
features in a second silicon substrate, the second set of device
features being configured to cooperate with the first set of device
features when joined therewith; and the first and second silicon
substrates being permanently joined with the first and second sets
of cooperating features being mutually aligned.
[0034] FIG. 2 illustrates the method of the invention embodied in a
process flow diagram 75. According to the method of the invention
an array of first 3-dimensional micromechanical device features is
micromachined in a first silicon wafer, and a first optical
alignment mark is micromachined on a face of the first wafer
relative to the array of first device features. An array of second
3-dimensional micromechanical device features different from the
first device features is micromachined in a second silicon wafer,
the second 3-dimensional micromechanical device features being
arranged mate with cooperating first 3-dimensional micromechanical
device features micromachined in a first silicon wafer and being
configured to cooperate with the first 3-dimensional
micromechanical device features when joined therewith. A second
optical alignment mark is micromachined on a face of the second
wafer relative to the array of second device features. The first
and second wafers are prepared for wafer-to-wafer alignment and
high-temperature silicon fusion bonding in a conventional manner
according to one of the methods well-known and accepted in the
art.
[0035] The first and second arrays of device features are mounted
in a conventional align/bond apparatus of a type well-known in the
art and aligned as a function of aligning the first and second
optical alignment marks. The first and second arrays of device
features are thereafter initially temporarily and later permanently
joined by a conventional and well-known high-temperature silicon
fusion bonding process into an array of integral devices as a
function of permanently joining the first and second silicon wafers
into a single composite wafer. Using well-known conventional die
cutting techniques, the array of integral devices are subsequently
separated into individual devices each having the first and second
3-dimensional device features.
[0036] According to one embodiment of the invention, the method of
the invention is embodied in a pair of wafers having different but
cooperating first and second 3-dimensional features that, in
composite, form an array of acceleration sensors each having a
composite proof mass suspended by integral flexures from a
composite frame with one or more integral vibrating beams coupled
between the proof mass and frame electrical signal lines formed on
the vibrating beams and leading to one or more wire bond pads
formed on a surface of the composite frame, and an oscillation and
feed back circuit coupled to the vibrating beams through the wire
bond pads and electrical signal lines and interacting with a
magnetic field B for exciting the beams to resonance and for
receiving a signal representative of a variation in the resonant
frequency of the vibrating beams as a function of tension or
compression forces being applied by movement of the composite proof
mass relative to the composite frame.
[0037] FIG. 3 illustrates the method of the invention embodied in a
composite silicon wafer 100 formed of a pair of silicon wafers 102,
104 shown by example and without limitation as conventional round
wafers. Such wafers are also commonly provided in squares. The
wafers may be cut to standard round or square dimensions of
prescribed thickness, or alternatively, cut to special or
proprietary dimensions without affecting practice of the invention
as long as a conventional or other wafer align/bond machine can
operate with the wafers.
[0038] Each of the wafers 102, 104 includes first and second or top
and bottom substantially parallel surfaces spaced apart by the
thickness of the wafer material, which may be different for each of
the two wafers. Preferably, each of the top and bottom surfaces is
polished to a prescribed finish, with at least the mating surfaces,
i.e., the bottom of the top wafer 102 and the top of the bottom
wafer 104, being polished to a degree to accommodate and encourage
subsequent silicon fusion bonding.
[0039] Arrays of different first and second 3-dimensional
micromechanical device features 106 are micromachined in different
ones of the pair of silicon wafers 102, 104, such that, when the
wafers 102, 104 are joined together into the composite wafer 100,
an array of composite devices 108a-108n are provided in the wafer
100. The device features 106 are micromachined using conventional
microelectromechanical system, or "MEMS," fabrication techniques,
such as etching and laser cutting. The etching is, for example, a
conventional reactive ion etching (RIE) or a conventional deep
reaction ion etching (DRIE) technique, as are well-known in the
art.
[0040] In addition to the device features 106, one or more tool
marks 110 are provided on one surface of each of the pair of
silicon wafers 102, 104 to act as guides for aligning the
cooperating features 106 in the different wafers 102, 104 during
assembly. The marks 110 are, by example and without limitation,
formed by a laser or etch process before, after or simultaneously
with the device features 106 and are precisely aligned relative to
the features 106. The marks 110 are optical marks visible under
magnification as by use of a microscope, during mounting of the
wafers 102, 104 into a chuck or other holding apparatus of a
conventional wafer align/bond machine of a type well-known in the
art. The marks 110 thus operate in cooperation with the align/bond
machine to accurately align device features in the one of the
silicon wafers 102 with cooperating features in the other wafer 104
during assembly.
[0041] Accordingly, an array of the first 3-dimensional device
features 106 are micromachined into a quantity of the first wafers
102, and an array of the different second 3-dimensional device
features 106 are micromachined into a quantity of the second wafers
104. Each of the first and second wafers 102, 104 are also provided
with the tool marks 110.
[0042] Subsequent to micromaching the different arrays of
3-dimensional device features 106 in quantities of the first and
second silicon wafers 102, 104, the individual wafers are inspected
for quality of the features 106. If the yield is within prescribed
minimum limits, for example, a minimum of 70 percent yield of
useable features, the individual wafer is accepted for mating with
a wafer having an acceptable yield of the cooperating features 106.
Mating of a pair of the first and second wafers 102, 104 each
having acceptable yields is reasonably expected to produce a
composite wafer 100 having an acceptable yield of the composite
devices 108a-108n The wafers 102 having the first 3-dimensional
micromechanical device features 106 are thereafter interchangeable.
The wafers 104 having the second 3-dimensional micromechanical
device features 106 are also thereafter interchangeable. This
interchangeability eliminates any requirement to further inspect
the features 106 or to match specific first wafers 102 with
specific second wafers 104. In other words, the selection of
matching parts that is so common in high precision micromechanical
devices is completely eliminated by the method of the
invention.
[0043] Any one of the quantity of accepted first wafers 102 with
any one of the quantity of accepted second wafers 104 as follows.
The surfaces of the two wafers 102, 104 intended for mating are
cleaned in a manner expected to produce a silicon fusion bond. See,
for example, the cleaning method described by Canaperi, et al. in
U.S. Pat. No. 6,475,072, "METHOD OF WAFER SMOOTHING FOR BONDING
USING CHEMO-MECHANICAL POLISHING (CMP)," the complete disclosure of
which is incorporated herein by reference, which is performed with
a down force of 1 psi, a backside air pressure of 0.5 psi, a platen
speed of 50 rpm, a carrier speed of 30 rpm and a slurry flow rate
of 140 milliliters per minute. Other appropriate cleaning methods
are also well-known in the art. For example, the previously
polished mating surfaces. i.e., the bottom of the top wafer 102 and
the top of the bottom wafer 104, are cleaned and primed to a degree
to provide surface sufficient to accommodate and encourage
subsequent silicon fusion bonding. The priming process generates a
surface rich in OH ions, whereby van der Waals' forces can be used
to temporarily join the wafers 102, 104.
[0044] The individual cleaned and primed wafers 102, 104 are
mounted in the wafer align/bond machine and aligned relative to one
another using the one or more optical tool marks 110 provided in
each of the wafers. The mutually aligned wafers 102, 104 are
brought together under vacuum by the align/bond machine whereupon
they are temporarily joined by the van der Waals' interaction.
[0045] The temporarily joined wafers 102, 104 are high-temperature
annealed in a conventional silicon fusion bonding oven of a type
well-known in the art wherein a fusion bond joint 112 is formed
between the wafers 102, 104. Upon release from the fusion bonding
process, the wafers 102, 104 are fused into a single integral
composite wafer 100 having the first and second 3-dimensional
micromechanical device features 106 micromachined in the respective
first and second sides indicated generally at 102, 104. The
individual free-standing composite 3-dimensional micromechanical
devices 108a-108n are released from the composite wafer 100 having
the first and second 3-dimensional micromechanical device features
106 aligned in a cooperative manner. One such wafer aligning and
bonding process is described by Bower, et al. in U.S. Pat. No.
5,236,118, "ALIGNED WAFER BONDING," the complete disclosure of
which is incorporated herein by reference, for forming
3-dimensional structures in separate prefabricated layers rather
than monolithically in a single layer. Another wafer aligning and
bonding apparatus and method is described by Collins, et al. in
U.S. Pat. No. 5,545,283, "APPARATUS FOR BONDING WAFER PAIRS," the
complete disclosure of which is incorporated herein by reference,
having a heated platen surrounded by a pressurization vessel
wherein the heated platen includes channels connected to a hole
which runs through it. Wafer pairs are placed on the top surface of
the platen, a rubber mat is placed over the top of the wafer pairs,
and a vacuum is drawn through the hole and the channels. The rubber
mat compresses the wafer pairs. The platen is heated for the
bonding process. The pressurization chamber is pressurized
supplying additional bonding pressure to the wafer pair. Once
sufficiently heated, the heated platen is liquid cooled completing
the bonding process. Other appropriate wafer aligning and bonding
apparatus and methods are also well-known in the art.
[0046] FIG. 4 illustrates an exemplary composite device 108a formed
using the method of the invention as describe herein. Accordingly,
each device 108a includes a first substrate 114 having
micromachined therein a first partial pendulum or proof mass 116
connected to a first partial frame 118 by one or more complete
stand-alone flexures 120 for translation or rotation about a hinge
axis HA defined by the one or more flexures 120. Each device 108a
also includes a second substrate 122 having micromachined therein a
second partial pendulum or proof mass 124 connected to a second
partial frame 126 and one or more complete stand-alone vibrating
beams 128 coupled between the second partial proof mass 124 and the
second partial frame 126, and wherein the first and second partial
proof masses 116 and 124 are permanently joined together, by
example and without limitation using high-temperature silicon
fusion bonding, into a single composite pendulum or proof mass 130
and the first and second partial frames 118 and 126 are
substantially simultaneously permanently joined together into a
single composite frame 132 such that the composite proof mass 130
is suspended from the composite frame 132 by the one or more
flexures 120 for translation or rotation about the hinge axis HA,
and the one or more vibrating beams 128 are coupled between the
composite proof mass 130 and the composite frame 132 for having
tension or compression forces applied thereto as a function of the
composite proof mass 130 moving relative to the composite frame
132. The one or more vibrating beams 128 are optionally configured
as either two-bar or four-bar double-ended tuning forks (DETFs) as
are well-known in the art. For example, the DETFs 128 are four-bar
DETFs of the type described in incorporated U.S. Pat. No. 5,501,103
with the outer pairs being mechanically coupled by respective cross
members 134, 136 that cause these beam pairs to vibrate
together.
[0047] In practice, each individual composite device 108a-108n is
provided with means for vibrating the one or more DETFs 128 at
their respective resonant frequency when the composite proof mass
130 is at rest. For example, electrical conductors are provided on
the beams of the DETFs 128 and electrically interconnected to an
oscillator circuit of the type described herein is provided
interact with a magnetic field to drive the outer set of vibrating
beams. The driven outer set of vibrating beams in turn drive the
inner set of vibrating beams. An output or pickoff voltage is
generated across the inner set of beams, which is then applied to
an amplifier by way of a positive feedback loop in order to form
the oscillator.
[0048] FIG. 5 shows the first portion of the method of the
invention embodied in a the first substrate 114 of the exemplary
composite device 108a that includes the first partial proof mass
116 connected to the first Dartial frame 118 by the one or more
flexures 120 for translation or rotation about the hinge axis HA.
Each first substrate 114 also includes a cooperating relief area
138 positioned to provide operating space for each of the one or
more vibrating beams 128 provided in the second substrate 122. As
discussed above, an array of such first substrates 114 is
micromachined in one of the pair of silicon wafers 102, 104 shown
in FIG. 3 as 3-dimensional device features 106 and form one part of
the composite MEMS accelerometer devices 108a-108n.
[0049] FIG. 6 shows the first portion of the method of the
invention embodied in a the second substrate 122 of the exemplary
composite device 108a that includes the second partial proof mass
124 connected to the second partial frame 126 by the one or more
vibrating beams 128. As discussed above, an array of such second
substrates 122 is micromachined in a different one of the pair of
silicon wafers 102, 104 shown in FIG. 3 as 3-dimensional device
features 106 and form a second part of the composite MEMS
accelerometer devices 108a-108n. The array of second partial proof
masses 124 and second partial frames 126 are arranged to mate with
the array of first partial proof masses 116 and first partial
frames 118 to form an array of the composite proof masses 130 and
frames 132 shown in FIG. 4. Similarly, the array of vibrating beams
128 is arranged to mate with the array of cooperating relief area
138 formed in the first substrates 114 such that during operation
the vibrating beams 128 have sufficient space in which to
operate.
[0050] While the preferred embodiment of the invention has been
illustrated and described, it will be appreciated that various
changes can be made therein without departing from the spirit and
scope of the invention.
* * * * *